Such a differential mixer is already known in the art, e.g. under the form of a Gilbert cell as described in the book "Analysis and design of analog integrated circuits" of P. R. Gray and R. G. Meyer, published by J. Wiley & Sons, New York, 1977, and more particularly on pages 563 to 575 thereof. This known mixer comprises, between its voltage supply terminals, three circuit levels:
a first input level including the pair of cross-linked differential amplifiers to which the first input voltage is applied;
a second input level including a differential amplifier to which the second input voltage is applied and which provides the differential current; and
a bias level including the current source.
Over the past few years, a Gilbert cell generally built up with bipolar switching transistors suited perfectly to be used as mixer or multiplier in GSM (Global System for Mobile communications) or other radio transmitters operating with a 5 Volt supply. Today however, silicon bipolar designers are struggling for the next step: a 3 Volt supply in battery operated wireless phones. The 3 Volt supply, with a tolerance of.+-.10%, allows to reduce the number of batteries, the cost, the volume and the weight of the handset.
Since a silicon bipolar switching transistors has an emitter-to-collector voltage drop of at least 1 Volt, the overall voltage drop across the above three circuit levels of a Gilbert cell is at least equal to 3 Volt. The known Gilbert cell can thus no longer be used with a 3 Volt supply because this leaves no headroom for the signal, especially when the supply voltage drops towards 2.7 Volt at the end of the battery cycle.